1. Field of the Invention
This invention relates in general to electric submersible pumps (ESPs) and, in particular, to a method for detecting and preventing gas lock based on vibration of the ESP system.
2. Brief Description of Related Art
Electric submersible pump (ESP) assemblies are disposed within wellbores and operate immersed in wellbore fluids. The ESP assemblies generally include a pump, a seal section, and a submersible motor. Generally, the motor is downhole from the pump, and the seal section connects the motor and the pump. The motor rotates the shaft that, in turn, rotates components within the pump to lift fluid through a production tubing string to the surface. The seal section provides an area for the expansion of the ESP motor oil volume, equalizes the internal unit pressure with the wellbore annulus pressures, isolates the clean motor oil from wellbore fluids to prevent contamination, and supports the pump shaft thrust load. The pump has a pump intake proximate to the seal section that allows wellbore fluid into the pump. The pump also includes at least one impeller and at least one diffuser positioned within the pump to lift the wellbore fluid to the surface. The pump further includes a pump discharge that connects to a production tubing string to transfer wellbore fluids, pressurized in the pump, to a desired location. The pump intake may be an integral part of the pump, or the pump intake may be a separate component mechanically and rotatably connected to the pump between the pump and seal section.
In some embodiments, the ESP assembly includes a gas separator positioned between the seal section and the pump. ESPs are designed to handle liquid and will suffer from head degradation and gas locking in the presence of a high percentage of free gas. The gas separator is installed at the pump intake, between the seal section and the pump. Wellbore fluid enters the gas separator and passes through the gas separator into the pump intake. The wellbore fluid is rotated within the separator, centrifugally separating heavier wellbore fluid from lighter wellbore fluid. Generally, heavier wellbore fluid corresponds with fluid that has a lower gas content, and lighter wellbore fluid corresponds with fluid having a higher gas content. The gas separator then directs the heavier wellbore fluid to the pump section intake and the lighter wellbore fluid back into the annulus of the casing. Despite use of gas separators, gas lock may occur when an ESP ingests sufficient gas so that the ESP can no longer pump fluid to the surface due to, for example, large gas bubbles in the well fluid. This occurs despite the use of gas separators, Failure to resolve a gas-locked ESP can result in overheating and premature failure.
Conventional practice on an ESP is to set a low threshold on motor current to determine when the pump is in gas lock. When the motor pulls less current than specified to rotate the pump impellers due to gas lock, the threshold is reached. When this threshold is crossed, the pump is typically stopped and a restart is not attempted until the fluid column in the production tubing has dissipated through the pump. This wait time represents lost production. However, the current detection method may prove unreliable, potentially giving false positives. In other situations, current detection may not provide an indication of impending gas lock until after the gas lock condition manifests in the pump. Therefore, an improved system for detecting impending gas lock and adjusting pump operating conditions to prevent onset of gas lock is desired.